In semiconductor processing, photomasks are used in photolithographic processes to define circuit structures on semiconductor substrates. Such masks are protected from environmental contamination and other effects by thin membrane pellicles. Commonly, such pellicles are formed of very thin membranes of organic material. Typically, such thin pellicles are on the order of less than about two micron (μm) thick. With the ever increasing drive toward smaller features sizes and increasing circuit densities, the industry is driven toward the need to obtain higher resolution in the transfer of mask patterns onto semiconductor substrates. One means of achieving this increased resolution is through the use of shorter wavelength exposure sources. One exposure source coming into ever increasing use is the deep ultraviolet (DUV) laser. Typical examples of such lasers are ArF (argon fluoride) lasers and F2 (fluorine) lasers. In common implementation, the F2 laser generates an exposing light beam having a wavelength of 157 nm (nanometers).
Such short wavelength exposure sources can damage conventional thin membrane pellicles after only a few exposures to 157 nm light at typical exposure levels. As reliance on 157 nm exposure sources increases, the traditional thin membrane organic pellicles currently used to protect the mask surface can no longer be used. Consequently, the industry is developing thick fused silica pellicles as an alternative. As used herein, “thick pellicles” are defined as pellicles thicker than about 2 μm thick (particularly, 300 μm and 800 μm thick pellicles). Industry organizations such as SEMATECH and its Japanese analog SELETE have called for the use of 800 μm fused silica pellicles having a thickness tolerance of about ±0.5 μm. Although these thick pellicles are more rugged in the face of UV exposure, the use of these new thicker pellicles presents significant optical problems for the conventional optical systems used in current photomask inspection tools.
In conventional optical systems and inspection tools, the thin organic pellicles are so thin as to be optically insignificant to the optical system, and, in general, can be ignored. For example, in inspection systems having numerical apertures (NA) of 0.8 or less, pellicle thicknesses of 2 μm or thinner have negligible effect on system optical performance, and can be ignored.
In contrast, the thick pellicles proposed by SELETE and SEMATECH will induce significant optical effects that must be corrected in order to obtain satisfactory resolution.
To resolve very small features on a photomask requires a very high resolution imaging system. The resolution of an optical system can be represented by 2*NA/λ, where NA is numerical aperture, and λ is the wavelength of light. Thus, to increase resolution (i.e. to see smaller defects), NA can be increased, λ can be decreased, or both may occur. For high-resolution photomask inspection stations, NA can be pushed beyond 0.8, and λ can be decreased into the UV and DUV regions of the spectrum. However, to detect the smallest of defects under these conditions, these imaging systems must have P-V wavefront errors of well under λ/4 (the well-known Raleigh Criteria) where λ is the wavelength of light used to image the defects. Therefore, the highly aberrated wavefront that results from passing a beam of light through an 800 μm thick pellicle at high NA values, must be corrected. Without such correction, the image quality would be so poor that even large, high contrast defects would be missed during inspection. Thus, it is important that the industry find a solution to this very serious problem.
FIGS. 1(a)-1(c) present a simplified illustration of one aspect of the problems introduced by using thick pellicles. FIG. 1(a) is a simplified and schematic depiction of a generic conventional objective lens system 101. A light beam 102 is passed through the objective lens system 101 where it becomes focused at a point 103 in the image plane 104 of the objective lens system 101. Commonly, many optical elements are used by the objective lens system 101 to correct for a variety of optical aberrations to accomplish the needs of the objective lens system 101. Such optical elements commonly include lenses, lens groups, gratings, apertures, filters, as well as a number of other optical devices known to those having ordinary skill in the art. In a surface inspection tool, an object 105 (e.g., a photomask) being inspected is positioned at the image plane 104 for inspection by the tool.
FIG. 1(b) depicts the same objective lens system 101 as shown in FIG. 1(a). A conventional thin membrane pellicle 106 (e.g., pellicles having a thickness of 2 μm or less) is interposed between the objective lens system 101 and the object 105. As is depicted, the light beam 102 is passed through the objective lens system 101 and is focused at point 103 in the image plane 104. The presence of the thin pellicle 106 has a negligible effect on the light beam. As a result, until now there has not been a need to address the optical effects induced by the presence (or absence) of pellicles.
FIG. 1(c) is a simplified and schematic illustration depicting some of the problems induced by the interposition of a thick pellicle 107 between a conventional objective lens system 101 and an inspection surface (not shown in this view). The light beam 102 passes through the objective lens system 101 onto the thick pellicle 107. The thick pellicle 107 functions as an aberration inducing optical element. An aberration so produced can be generally described as an aperture dependent focus, which results from a beam 102 passing through the thick pellicle 107. This is illustrated in FIG. 1(c) using a few example aperture locations and resultant focal points (110, 111, 112). The presence of the thick pellicle aberrates the light beam 102 such that there is no single focal plane where the parts of the incident beam entering different parts of the aperture, are all in focus. The end result is a blurry, low contrast image. Such a distorted light beam cannot be used to effectively image small defects on a photomask surface. Thus, solutions to this problem are needed.
In particular, there is a need for a lens system (and accompanying inspection tool) capable of inspecting objects through both thick and thin pellicles (or in the absence of pellicles). In some embodiments, the system should also be capable of obtaining a high numerical aperture (NA) and a relatively long working distance. Moreover, it is especially advantageous for such a system to achieve such inspection flexibility by changing, moving, adding or removing only a few optical elements.